336 
PHYSICS: C. BARUS 
mirrors m, m\ rotates at pleasure in the vertical arms A, A' oi square 
brass tube. A, A' in turn may be slightly swivelled about the horizontal 
axis by b\ in a rigid lateral projection of the rings r, r\ Thus w' are 
capable of rotation around 3 axes normal to each other, and adequately 
clamped in any position. 
The component ray HN may be adjusted to the center of the lower 
mirror m' by placing the collimator L and then guided axially by m'mN 
as described each being adjustable. The component ray EM may be 
similarly adjusted to the center of the lower mirror m' (at 45°) by (slight- 
ly) rotating the half silver plate, H (on horizontal and vertical axes) 
and then guided axially by m'mM. As a whole the adjustment is diffi- 
cult, though it need not be much refined. Clear white sHt images in 
the telescope T are an adequate criterion. 
In the absence of a Hquid in CC figure 2, the fringes are easily found 
after careful preliminary measurement and they are strong and satis- 
factory. When this adjustment is given the presence of Hquid in CC, 
if the two columns are of nearly equal length, does not much modify the 
adjustment. In fact the fringes were found much more easily than I 
anticipated and in quiet surroundings they are strong and fine. It is 
* necessary however that the tube CC should be of sufficient width to avoid 
all curvature due to capillarity, at least in the axis. Tubes 2 cm. in 
diameter and 10 cm. long of thin brass were first tried, but proved to 
be too narrow. No sharp sHt images could be obtained with reasonable 
care as to setting the mirrors. Thereafter tubes 4 cm. in, diameter were 
used and these proved to be satisfactory. SHt images were sharp and 
parallel and could be easily brought to coincide. 
3. Equations. — Some estimate of the increments to be anticipated 
may be given here, and expressed in terms of the Dulong-Petit experi- 
ment. If a is the mean coefficient of expansion of water at the tem- 
perature in question and AjET the increment of the head H correspond- 
ing to the temperature difference A/ between the colimins 
AH ^ a HM (1) 
Again if AiV corresponding to AH is the displacement of center of elHpses 
at the wave length X and ju the index of refraction of water, so that ^ = 
A + B/\\ nearly. 
Hence A t may be computed as 
_ Aw x^v 
^ ~ (m - 1 -f 2 B/\^) aH ^ ^ 
